Certain impurities in a material give rise to localized energy levels in the material which can trap either electrons or holes. These localized energy levels, or traps, lie relatively deep in the forbidden energy band gap of the material as compared with the relatively shallow levels resulting from the intentional introduction of donors or acceptors in the material due to other types of impurities.
It is well known in the art that by measurement of the capacitance change versus time of the depletion region, or space charge layer near rectifying junctions in semiconductor devices, the activation energy (energy level; E.sub.T), the density (N.sub.T), and the capture cross-section (.sigma.; the capture rate is a constant times .sigma.) of traps in the band gap can be determined. In other words, the change of the occupancy of the localized states in the forbidden gap can be measured as a change in capacitance. Deep level transient spectroscopy (DLTS) is one of the most powerful tools for measuring these localized states. In DLTS systems, the localized states are periodically filled and emptied, and the amplitude of the resulting periodic capacitance change is determined by the concentration of the energy states, while the kinetics thereof are determined by the activation energies and thermal capture cross-sections of the localized states as a function of temperature. The occupancy of the localized states are changed by pulses, such as voltage, current, or light pulses, which are applied to a rectifying junction in the material. The DLTS method and the instruments utilizing this method are indispensable factors of development and quality control in the field of semiconductor devices.
Conventional DLTS methods generally involve constructing and analyzing a spectrum obtained from capacitance transients over a temperature range. Well known methods of constructing these spectrums include the boxcar technique, the rectangular ("lockin style") technique, and the Miller correlator technique, all as described in for example, Miller et seq., "Capacitance Transient Spectroscopy," Ann. Rev. Metr. Sci., pp. 348-377 (1977).
For further discussions in regard to the DLTS methods, see A. Rohatgi, "Application of DLTS Technique for Study of Junctions and Interfaces," Proceedings of the National Vacuum Symposium, pp. 115-133 (December 1979) and Miller et seq., "Capacitance Transient Spectroscopy," Ann. Rev. Metr. Sci., pp. 348-377 (1977). Furthermore, known patents which provide systems and methods for DLTS include U.S. Pat. No. 5,047,713 to Kirino et al., U.S. Pat. No. 4,839,588 to Jantsch et al., U.S. Pat. No. 4,571,541 to Frenczi et al., U.S. Pat. No. 4,437,060 to Frenczi et al., U.S. Pat. No. 4,208,624 to Miller, U.S. Pat. No. 3,943,442 to Fletcher et al., U.S. Pat. No. 3,859,595 to Lang, and U.S. Pat. No. 3,605,015 to Copeland.
Many computer-controlled systems for performing DLTS techniques are also known in the art. However, these computer-controlled systems are generally expensive and are difficult to build. Additionally, conventional DLTS techniques suffer from at least one very important limitation: they cannot resolve closely spaced energy levels E.sub.T or distributed energy levels E.sub.T within the band gap. In recent years, many researchers have attempted to extend the power of DLTS beyond this inherent limitation by using different types of transient analyses, including nonlinear least squares fitting, spectral analysis DLTS (SADLTS), fast fourier transform (FFT), method of moments, correlation method of linear predictive modeling, modulation function methods, and mixed methods. However, most commercial DLTS systems do not lend themselves to digitization of the transients without some modifications that can be, at best, somewhat cumbersome and, at worst, very restrictive. Specifically, it is difficult and/or time consuming to observe the transient over many decades of time without manual intervention due to the nonintegrated nature of the pulse generation and sampling processes.
Thus, a heretofore unaddressed need exists in the industry for an inexpensive and flexible computer-controlled DLTS system and method for quickly and accurately measuring characteristics of traps within a material, including the activation energy (E.sub.T), the density (N.sub.T), and the capture cross-section (.sigma.) of the traps, using transient analysis.